The present invention relates to an in-furnace slag level measuring apparatus for measuring the level of in-furnace slag, melt, raw material, etc. in a convertor or the like by utilizing a microwave.
In a conventional in-furnace slag level measuring apparatus, for example, a microwave radar is provided at a furnace top of a convertor so that electromagnetic wave transmitted by the microwave radar through a waveguide and a transmission antenna is reflected on a slag surface. The electromagnetic wave thus reflected is received through a reception antenna and a waveguide and then subjected to signal processing to measure the distance between the microwave radar and the in-furnace. slag surface.
As the microwave radar for use for such measurement, there are various kinds of measurement systems as follows.
One of those systems is, for example, an FM-CW microwave radar system. As disclosed, for example, in Japanese Patent Unexamined Publication No. Sho-63-21584, this measurement system comprises the steps of: transmitting a microwave formed by frequency-modulating a continuous microwave of about 10 GHz, from an antenna toward a level surface; and counting a beat frequency produced by mixing the transmission signal with the wave reflected on the level surface to thereby measure the distance between the microwave radar and the slag surface. That is, the distance measurement in this measuring system is based on the fact that the propagation time of the microwave required for reciprocating the distance between the antenna and the slag surface corresponds to the aforementioned beat frequency.
As the system of this type, there is a pulse-modulated microwave radar system. This measurement system comprises steps of: pulse-modulating a microwave having a frequency of about 10 GHz to about 20 GHz and transmitting the pulse-modulated microwave as in an ordinary airplane; radar and measuring the distance between the radar and the slag surface on the basis of the fact that the propagation time of the microwave required for receiving the microwave reflected on the slag surface is in proportion to the distance between the radar and the level surface.
In the conventional in-furnace slag level measuring apparatus using such a microwave radar, the transmission antenna and the reception antenna are fixed to specific positions at the furnace top portion of the convertor or in the furnace. Accordingly, there arise the following problems as to the change of the slag level in the furnace.
In the case where the transmission and reception antennas are fixed at the furnace top portion, the slag level position cannot be often measured accurately because of the influence of unnecessary signals reflected from the in-furnace lance, the furance opening portion and the furnace wall portion when the distance between the transmission and reception antennas and the slag level increases as the slag level in the furnace decreases. In order to solve the above-mentioned problems, on the other hand, the inventors of this application have proposed, Japanese Patent Unexamined Publication No. Hei-2-98685, a distance measuring method of the microwave M-type radar system utilizing an M-type signal which is a kind of a pseudo random signal. In this measuring method, a time difference between a time-series pattern obtained by detecting a carrier which is phase-modulated with a first pseudo random signal, transmitted toward a target and then reflected from the target, and another time-series pattern of a product obtained by directly multiplying the first pseudo signal by a second pseudo random signal is detected to thereby measure the distance between the measuring apparatus and the slag surface which is the target.
In the in-furnace slag level measuring apparatus using the above-mentioned random signal processing, the distance between the antenna and the in-furnace slag level surface is obtained form the time interval between the respective peaks of a detection signal and a time reference signal with a preferable result. In the apparatus, however, there are problems as follows.
In a convertor or the like, the change of slag in the furnace is so large that the change of the distance between the antenna and the slag surface and the change of the shape of the slag surface are also large, and in addition, the signal strength of the reception microwave signal reflected from the slag surface is also largely changed by the scattering of slag or the like. In the case where the reflected signal strength becomes large, accordingly, signal saturation occurs in an amplifier in a radar apparatus to cause distortion in a detection signal waveform obtained as a result of signal processing to make it difficult to correctly detect the peak of the detection signal pulse to thereby produce a measurement error. In the case where the reflected signal strength becomes small, on the contrary, the detection signal output also becomes small to lower the signal to noise ratio S/N so that an erroneous signal peak is sometimes detected to thereby produce a measurement error.